This invention relates to a system and a method for controlling a transfer case and more particularly, to a system and a method for controlling an automatic four-wheel drive transfer case which detects a condition of excessive heat generation within the transfer case, and which shifts the transfer case into a locked four-wheel drive mode in response to such a detection, thereby preventing potential damage and wear to the transfer case clutch.
Four-wheel drive vehicles typically include a transfer case which selectively transfers torque and power from the vehicle""s input shaft to a front and a rear driveshaft, thereby selectively rotating the front and rear driveshafts.
In four-wheel drive systems having an xe2x80x9cautomaticxe2x80x9d or xe2x80x9con-demandxe2x80x9d drive mode, both the front and rear pairs of wheels are continually in drivable engagement with the transfer case, but substantially all of the torque is transferred to one pair of wheels when no slippage is detected. When a slippage is detected, the transfer case selectively transfers torque from a primary driveshaft (e.g., the rear driveshaft) to a secondary driveshaft (e.g., the front driveshaft), thereby reducing the torque provided to the primary driveshaft and increasing the torque provided to the secondary driveshaft. Hence, in this type of system, torque is transferred in a mode substantially similar to a two-wheel drive vehicle until slippage is detected.
A controller and/or control system determines the amount of torque that is to be provided to the rear and front driveshafts (e.g., primary and secondary driveshafts) based upon vehicle sensory data, and generates a control or command signal to the transfer case, thereby controlling the amount of torque which is provided to the rear and front driveshafts.
For example and without limitation, when the speed of the front driveshaft or front wheels differs from the speed of the rear driveshaft or rear wheels by a certain amount or xe2x80x9cerrorxe2x80x9d, these prior systems alter or change the value of the control or command signal to provide more or less torque to the front and/or to the rear driveshaft in order to substantially prevent and/or reduce the likelihood of relative slip between the front and rear wheels. While these types of prior systems relatively quickly respond to and correct relative slip conditions between the front and rear wheels in many situations, they suffer from some drawbacks.
One drawback associated with these prior systems is that the control systems often cause excessive clutch cycling when the vehicle is being operated in conditions where the wheels experience slippage for an extended period of time, such as in sand, mud, or deep snow. This excessive clutch cycling generates relatively high amounts of heat which can cause excessive wear and potential damage to the clutch components.
Efforts have been made to reduce excessive clutch cycling in these situations by monitoring the clutch duty cycle and causing the transfer case to enter into a xe2x80x9clockedxe2x80x9d four-wheel drive operating mode (e.g., a four-wheel high mode) if the clutch duty cycle waveform corresponds to an excessive cycling condition. One drawback with this type of system is that it does not accurately account for or reflect the amount of heat generated within the transfer case clutch, which is a primary cause of clutch wear and damage. Particularly, since these types of prior systems monitor only clutch duty cycle, they do not properly account for the degree of slip and loading conditions which are determinative of the amount of heat generated by the clutch.
There is therefore a need for a system and method for controlling a transfer case within an automatic four-wheel drive vehicle which utilizes a thermal model to detect high load and high slip conditions, and to switch the transfer case into a xe2x80x9clockedxe2x80x9d four-wheel high operating mode in response to such a detection, thereby preventing excessive wear and potential damage to the transfer case clutch.
It is a first object of the invention to provide a system and method for controlling a transfer case within a four-wheel drive vehicle which overcomes at least some of the previously delineated drawbacks of the prior systems, devices, and/or methods.
It is a second object of the invention to provide a system and method for controlling a transfer case within an automatic four-wheel drive vehicle which accurately detects high load and high slip conditions and which is effective to switch the transfer case into a xe2x80x9clockedxe2x80x9d four-wheel drive operating mode in response to such a detection, thereby preventing excessive wear and potential damage to the transfer case clutch.
It is a third object of the invention to provide a system and a method for controlling a transfer case within an automatic four-wheel drive vehicle which utilizes a thermal model to detect high load and high slip conditions and which is effective to switch the vehicle""s transfer case into a xe2x80x9clockedxe2x80x9d four-wheel drive operating mode in response to such a detection.
According to a first aspect of the present invention a system is provided for controlling a transfer case. The transfer case is operatively disposed within a four-wheel drive vehicle of the type having a selectively positionable throttle, a front driveshaft which rotates at a first speed, and a rear driveshaft which rotates at a second speed. The transfer case is also adapted to selectively provide torque to the front and the rear driveshaft and to selectively operate in an automatic four-wheel drive mode and in a locked four-wheel drive mode. The system includes a first sensor which measures the first speed and which generates a first signal representing the first speed; a second sensor which measures the second speed and which generates a second signal representing the second speed; a third sensor which measures a position of the throttle and which generates a third signal representing the throttle position; and a controller which is communicatively coupled to the transfer case, to the first sensor, and to the second sensor, which receives the first, second, and third speeds, which utilizes the first signal and the second signal to calculate a delta shaft speed and which utilizes the delta shaft speed and the third signal to estimate an amount of heat generated by the clutch, the controller being further effective, when operating in the automatic four-wheel drive mode, to automatically switch from the automatic four-wheel drive mode to the locked four-wheel drive mode when the estimated amount of heat exceeds a predetermined threshold value.
According to a second aspect of the present invention, a method is provided for controlling a transfer case which is operatively disposed within a four-wheel drive vehicle and which includes a clutch which selectively transfers torque to a front driveshaft and a rear driveshaft, the transfer case being operable in an automatic four-wheel drive mode and a locked four-wheel drive mode. The method includes the steps of: monitoring a delta shaft speed; monitoring a position of the throttle; estimating an amount of heat generated in the clutch by integrating a function which is based upon the delta shaft speed and the position of the throttle over a predetermined number of consecutive intervals of time; determining if the estimated amount of heat exceeds a predetermined threshold value over each of the consecutive intervals of time; and automatically switching the transfer case into the locked four-wheel drive mode if the estimated amount of heat exceeds the predetermined threshold value over each of the consecutive intervals of time.
Further objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred embodiment of the invention and by reference to the following drawings.